Take a look at Tektro R556 long-reach calipers. I found out about them when I was thinking about setting up a bike as a 650B conversion (I'd already found a long-enough reach brake for a 27"-700c fg/ss conversion). They definitely have the reach you want. Some folks think they flex a bit due to the long caliper arms. A number of on-line bike shops have them (bikeman.com in particular) and I see them on ebay frequently. They use a recessed nut, so you may have to drill out the back hole in the fork slightly.